1. Field of the Invention
The present invention relates to a power supply unit provided with an output monitoring function. In particular the invention relates to a power supply unit with a fail-safe construction which can stably supply an output of a level within a predetermined range while monitoring the output, and which can stop the output at the time of an abnormality.
2. Description of the Related Art
Heretofore, as a direct current power supply unit, a switching power supply unit is commonly used, since this has the advantage of high efficiency, small size, light weight and so on.
This switching power supply unit will be simply described.
With this unit, an input commercial AC (alternating current) power supply is converted to DC (direct current) in a smoothing circuit comprising a rectifying circuit and a smoothing capacitor, and then supplied to a primary side of a high frequency transformer. The DC supplied to the primary side is converted to AC by ON/OFF switching of a switching element, transmitted to a secondary side of the transformer, converted to a DC output by the smoothing circuit and then supplied to a load. Moreover, this DC output is fed back to a switching control circuit for controlling the switching element. The switching control circuit incorporates an error sampling circuit, an oscillation circuit, and a pulse width modulation circuit, and detects an error between the fed back DC output and a previously set target value for the output with the error sampling circuit, carries out pulse width modulation (PWM) on a signal of the oscillation circuit with the pulse width modulation circuit in accordance with the detected error and controls the duty ratio of a PWM signal, and controls the ON/OFF period of the switching element, to thereby stabilize the target DC output and supply this to the load.
In the case of a switching power supply unit fitted with such an output monitoring function, if for example a fault occurs such that the DC output is not fed back to the switching control circuit, the switching control circuit judges the input to be zero and thus increases the output. Moreover, if a disconnection fault occurs in the primary side smoothing capacitor, the primary side voltage of the transformer periodically becomes a low level and this is reflected in the secondary side output. Furthermore, if an abnormality occurs in the secondary side smoothing capacitor, an increase of a high frequency ripple may occur.
However, with equipment demanding high safety, for example equipment used in the railway technology field, or for press machines or the like, then an erroneous operation attributed to an abnormality in the output from the power supply unit is extremely dangerous. Consequently, with power supply units it is necessary to be able to realize in a fail-safe manner, a characteristic that at the time of an output abnormality, operation is to the safe side (for example the output is stopped).
From this perspective, heretofore a technique for detecting an output abnormality to stop the power supply output of a switching power supply unit has been proposed for example in Japanese Unexamined Patent Publication Nos. 4-248317 and 10-336879.
In Japanese Unexamined Patent Publication No. 4-248317, there is shown a construction where a plurality of windings are provided in a high frequency transformer, and an output from a winding separate from a feedback control monitoring winding is monitored with an abnormality monitoring circuit, and the PWM signal is stopped based on the results of this monitoring, to thus stop the ON/OFF operation of the switching element so that the power supply output is stopped. Moreover, in Japanese Unexamined Patent Publication No. 10-336879, there is shown a construction where when the output voltage exceeds a previously set upper limit value, the output monitoring circuit outputs an abnormality signal to stop the ON/OFF operation of the switching element, and thus stop the power supply output.
However, these abnormality monitoring techniques, both only monitor for an overvoltage as output abnormality, and output abnormality on the low voltage side where the output level drops is not considered. With the conventional abnormality monitoring techniques, even if the output level drops with a circuit abnormality, this is not considered as an output abnormality, and the output is controlled to increase.
In order to detect a drop in output level as output abnormality, it is necessary to consider a time of power supply start-up. That is to say, since the output is zero at the time of power supply start-up and hence the abnormality monitoring circuit judges output abnormality and stops the switching operation of the primary side of the transformer, it is necessary to cancel the abnormality judgment in order to carry out the switching operation. However, a construction where at the time of power supply start-up, the power supply output is produced regardless of the monitoring result from the monitoring circuit, is not shown in the disclosures of Japanese Unexamined Patent Publication Nos. 4-248317 and 10-336879.
As an output error monitoring technique for a switching power supply unit, a technique which monitors not only for an overvoltage of the output but also for a drop in the output level, and stops the power supply output of the switching power supply unit at the time of output abnormality is proposed in Japanese Unexamined Patent Publication No. 7-95724.
With this technique, a pre-rectification intermediate output and a post rectification final output in the secondary side of the transformer are both monitored and when the two outputs are both abnormal overvoltages or are both abnormal low voltages, an abnormality detection signal indicating abnormality is output to an alarm monitoring circuit. The alarm monitoring circuit is constructed to, on input of the beforementioned abnormality detection signal, output a signal for stopping operation to the PWM control circuit to thereby stop the operation of the switching power supply unit.
However, in the case of Japanese Unexamined Patent Publication No. 7-95724, there is no disclosure of a specific construction for how the power supply output is started at the time of power supply start-up with the transformer secondary side output zero. With the construction of an embodiment circuit shown in FIG. 2 of this publication, it can be supposed that an abnormality detection output is generated from the low voltage monitoring circuit at the time of power supply start-up, and hence, the PWM control circuit is not operated due to the output from the alarm monitoring circuit, and the switching power supply unit can not be started. Furthermore, even if assumed that start-up of the power supply unit is possible, with the circuit construction of FIG. 2, a signal indicating abnormality is considered to be generated at a high level condition. In this case, if the transmission path for the abnormality detection signal is disconnected, the output abnormality cannot be notified, so that there is a danger that the power supply output cannot be stopped.
Also with an AC power supply, as with the DC power supply, there is a requirement for realizing in a fail-safe manner a characteristic where output abnormality is detected and operation is to the safe side (for example the output is stopped). However, heretofore such a technique for an AC power supply has not been set forth.
For example, with a sensor which receives an AC signal from an AC power supply and outputs an AC signal of a level corresponding to a sensing result of an object, in order for the sensor to have fail-safe characteristics, it is necessary for a level meaning safety to be higher than a level meaning danger. This will be clear if a fault involving a disconnection in the signal transmission circuit is considered. Moreover, in the case where the output level of the sensor is dependant on the input AC signal level and/or frequency, then in spite of a danger condition due to an error in the level and/or frequency of the AC power supply output for supply to the sensor, a safe condition may be erroneously indicated.
This problem will be explained taking the example of a temperature sensor with a transformer construction which uses for example a temperature sensitive magnetic material core.
A temperature sensitive magnetic material is a ferromagnetic material which is the same as a normal magnetic material at a previously determined set temperature (Curie temperature) or below, but with a rise in temperature, the saturation magnetic flux density thereof drops, and it becomes a paramagnetic material when exceeding the set temperature. With the detection temperature (core temperature) of the temperature sensor (transformer) equal to or less than the set temperature, the primary winding and the secondary winding of the temperature sensor are magnetically closely coupled by the core, and hence the AC signal from the AC power supply is transmitted to the secondary side. On the other hand, if the temperature exceeds the set temperature, the core becomes a paramagnetic material so that the primary winding and the secondary winding become substantially approximately air-core coupled and hence the AC signal level of the secondary winding drops significantly. The output from the temperature sensor (secondary side output of the transformer) is converted to DC by a rectifying circuit and then subjected to a threshold value operation with a level detection circuit, and when the operated value is equal to or above a predetermined level, an output of logic value=1 corresponding to a high energy condition indicating safety is generated. By means of such a temperature sensor, it is possible to notify a condition, with equal to or less than a predetermined temperature as a safe condition and more than the predetermined temperature as a danger condition.
However, even if the detection temperature (core temperature) of the temperature sensor exceeds the set temperature, the primary winding and the secondary winding are magnetically coupled though approximately air core coupled. Consequently, if the output level of the AC power supply increases or the frequency increases, the secondary side output level increases. If the secondary side output exceeds the threshold value, then in spite of the danger condition with the detection temperature exceeding the predetermined value, the output from the level detection circuit is logic value=1, that is, indicating safety.
Accordingly, in order to avoid the erroneous operation in such a sensor, there is provided a fail-safe AC power supply which can stably generate an AC output having a level within a predetermined range and which can stop the output at the time of an output abnormality is desired.
The present invention addresses the abovementioned situation with the object of providing a fail-safe power supply unit which can monitor an output level within an upper and lower limit threshold value range, and supply a power supply output only when an output is normal.
Accordingly, there is provided a power supply unit of the present invention, which switches a DC signal to produce an AC signal, and supplies to the outside an AC or DC power supply output based on the produced AC signal, comprising a monitoring circuit for monitoring as to whether or not the power supply output is within a previously set upper and lower limit threshold value range, and generating a normal verification signal of a high energy level indicating a normal condition when the power supply output is within the threshold value range, wherein the power supply output can be supplied to the outside when the normal verification signal is generated from the monitoring circuit.
With such a construction, the monitoring circuit can monitor as to whether or not the power supply output is within the upper and lower limit threshold value range, and can also monitor for a drop in the power supply output level. In particular, by generating the normal verification signal as a high energy level signal, a fail-safe construction can be realized.
In this case, if the construction is such that after an abnormality is detected and the normal verification signal is temporarily stopped, a normal verification signal is not generated from the monitoring circuit even if the abnormality is cancelled, then in the case where the normal verification signal is temporarily stopped with the occurrence of an abnormality at the time of power supply, then the supply of the power supply cannot be resumed unless for example a trigger is applied from the outside. Hence danger to an operator or the like, with the load being arbitrarily started with cancellation of the abnormality can be prevented.
With the present invention, there is provided a start circuit for generating a start signal for power supply output start-up. By means of this, power supply output can be started regardless of the output conditions of the monitoring circuit. In this case, a start signal may be generated for a predetermined time from a power supply start-up time, or a start signal may be generated for a predetermined time on input of a generation command signal from the outside after power supply start-up. Furthermore the construction may be such that when a normal verification signal is generated from the monitoring circuit, supply of the start signal is stopped.
With the present invention, the construction is such that there is provided a limited output generating circuit for generating at the time of power supply start-up, an output which is limited to a level within the upper and lower limit threshold value range, and the generated limited output is input to the monitoring circuit, and the normal verification signal is generated from the monitoring circuit.
With such a construction, since the power supply output generated from the limited output generating circuit at the time of power supply start-up is limited to a level within the upper and lower limit threshold value range, supply to the outside of an abnormal output at the time of power supply output start-up can be prevented.
With the present invention, the construction is such that a switch device is provided in an output supply path for supplying the power supply output to the outside, and the switch device is switched on by a normal verification signal from the monitoring circuit, to conduct an output supply path.
With such a construction, at the time of power supply start-up, the switch device is off so that the power supply output is not supplied to the outside. When the power supply output rises in the normal range, the switch device is switched on by the normal verification signal of the monitoring circuit so that the power supply output is supplied to the outside from the output supply path. Consequently, even if a start circuit is not provided, monitoring of the power supply output within the upper and lower limit threshold value range is possible. In this case, the switch device is constructed with a first switch and second switch connected in series, and if the construction is such that the first switch is switched on after the second switch is switched on, and is switched off prior to the second switch, then the fail-safe characteristic is improved by the double system construction for the switch device. Furthermore, this construction is advantageous in the case that relay contact points where fusion faults must be considered are used for this switch device.
The construction is such that the monitoring circuit is able to generate a normal verification signal under the condition that the first switch is off and the second switch is off.
With the power supply unit of the present invention provided with a first switching element for passing/interrupting the DC signal for input to a primary side of a transformer, and a switching control circuit for generating a control signal for ON/OFF switching of the first switching element, which generates a DC power supply output based on an AC output generated in a secondary side of the transformer with ON/OFF operation of the first switching element and supplies this to the outside, and at the same time inputs the power supply output to the switching control circuit and compares this with a target level set within the upper and lower limit threshold value range, and controls the control signal so that the power supply output level approaches the target level, to stabilize the power supply output, there is provided a start circuit for generating a start signal for starting power supply output, and when at least one of the start signal and the normal verification signal is being generated, the control signal of the switching control circuit is supplied to the first switching element. More specifically, the construction may be such that there is provided a logical sum device for generating a logical sum output for the normal verification signal and the start signal, and an output from the logical sum device is made a drive power supply of the switching control circuit. Furthermore, the construction may be such that a signal transmission device is disposed in a control signal supply path for supplying a control signal of the switching control circuit to the first switching element, for closing the control signal supply path when at least one signal of the start signal and the normal verification signal is generated, and transmitting the control signal to the first switching element.
With such a construction, in the DC switching power supply unit the start signal is generated to supply a control signal to the switching element to thereby start the power supply output, and if the power supply output is within a normal range after start-up, the power supply output can be maintained by the normal verification signal of the monitoring circuit.
With the present invention, the construction may be such that there is provided a limited output generating circuit for generating, when a start signal is generated from the start circuit, an output which is limited to a level within the upper and lower limit threshold value range, and the generated limited output is input to the monitoring circuit, and the normal verification signal is generated from the monitoring circuit.
With such a construction, since the power supply output generated from the limited output generating circuit based on the start signal is limited to the level within the upper and lower limit threshold value range, the supply of an abnormal output to the outside at the time of power supply output start-up by the start signal can be prevented. In this case, if the construction is such that the limited output of the limited output generating circuit is input to the monitoring circuit via the transformer, then a smoothing fault in the secondary side of the transformer can also be monitored. Moreover if the construction is such that a series circuit of a second switching element and an output limiting impedance element is connected in parallel with the first switching element which is series connected to the primary winding of the transformer, and there is provided a first and a second signal supply paths for respectively supplying the control signal of the switching control circuit to the first and second switching elements, and when the start signal is generated the second signal supply path is conducted to supply a control signal to the second switching element and a limited output is produced in the secondary side of the transformer, and when based on this limited output, the normal verification signal is generated from the monitoring circuit, the first signal supply path is conducted to supply a control signal to the first switching element, then even if the power consumption of the monitoring circuit fluctuates, the output can be stabilized, and also at the time of power supply output start-up, the performance of the switching control circuit can be checked.
With the present invention, the construction is such that a switch device is disposed in an output supply path for supplying the power supply output to the outside, and when a normal verification signal is generated from the monitoring circuit the output supply path is conducted by the switch device.
With such a construction, once the power supply output is within the normal range, the switch device is switched on so that the power supply output in the normal range is supplied to the outside. In this case, if there is a dual system with the switch device constructed with a plurality of switches connected in series, then the fail-safety characteristic is improved. Furthermore, if the construction is such that of the plurality of switches, one switch comes ON before the other switch and goes OFF after the other switch, then the one switch will not directly conduct/interrupt a current. Consequently, this construction is advantageous in the case that relay contact points where fusion faults must be considered are used for the switch.
With the present invention, the construction is such that there is provided a switch off verification circuit for detecting an OFF condition of the switch device and outputting an OFF verification signal, and the monitoring circuit is able to generate a normal verification signal on the condition that the OFF verification signal is being generated.
With such a construction, since a power supply output is generated after the OFF condition of the switch device is verified, then an ON fault of the switch device can be checked.
The monitoring circuit incorporates; a level verification device for verifying that a voltage level of the power supply output is within the upper and lower limit threshold value range, a ripple verification device for verifying that a ripple level of the power supply output is equal to or less than a predetermined level, a logical product device into which is input the outputs from the two verification devices, and an off-delay device for generating an output after a predetermined off-delay time lapse from input of the output from the logical product device, and the output from the off-delay device is made the normal verification signal.
With such a construction, when the voltage level and the ripple level of the power supply output are normal, a normal verification signal is generated and the power supply output is supplied. Then, even if a momentary load change occurs in the power supply output, if this load change is within the off-delay period of the off-delay device, the normal verification output from the off-delay device continues, and this is not regarded as an output abnormality.
The monitoring circuit incorporates; a level verification device for verifying that the voltage level of the power supply output is within the upper and lower limit threshold value range, a ripple verification device for verifying that a ripple level of the power supply output is equal to or less than a predetermined level, a logical product device into which is input the outputs from the two verification devices, an off-delay device for generating an output after a predetermined off-delay time lapse from input of the output from the logical product device, and an on-delay device which takes the output from the off-delay device and generates an output and stops the output after a predetermined on-delay time lapse from the output generation, and the one switch is driven based on the output from the off-delay device, and the output from the on-delay device is made the normal verification signal.
A monitoring circuit of this construction where the switch device is made up of a plurality of series connected switches, is an effective specific circuit in the case where one switch comes on before the other switches and goes off after the other switches.
The monitoring circuit incorporates; a level verification device for verifying that the voltage level of the power supply output is within the upper and lower limit threshold value range, a ripple verification device for verifying that a ripple level of the power supply output is equal to or less than a predetermined level, a logical product device into which is input the outputs from the two verification devices, an off-delay device for generating an output after a predetermined off-delay time lapse from input of the output from the logical product device, a self hold device with an output from the off-delay device input to a hold input terminal, and the off verification signal of the switch off verification circuit input to a trigger input terminal, which self holds a trigger input by its own output, and an on-delay device which takes the output from the off-delay device and generates an output and stops the output after a predetermined on-delay time lapse from the output generation, and the switch device is driven based on the output from the self hold device, and the output from the on-delay device is made the normal verification signal.
With such a construction, in the case where the OFF condition of the switch device is detected by the switch off verification circuit, and the construction is such that the monitoring circuit is able to generate a normal verification signal with the off verification of the switch device as a condition, an effective specific circuit is given. In this case, if the construction is such that a contact point which can be operated from the outside is disposed in an off verification signal supply path for connecting to the trigger input terminal of the self hold device, then after an output abnormality once occurs and the output supply is stopped, an output cannot be generated unless the contact point is again operated to ON from the outside.
The start circuit may be such as to generate a start signal for a predetermined time from the time of power supply start-up. Moreover, if the construction is such that when the normal verification signal is generated the supply of the start signal is stopped, the supply of an abnormal output to the outside based on the start signal can be prevented.
With a power supply unit of the present invention provided with a first switching element for passing/interrupting the DC signal for input to a primary side of a transformer, and a switching control circuit for generating a control signal for ON/OFF switching of the first switching element, which generates a DC power supply output based on an AC output generated in a secondary side of the transformer with ON/OFF operation of the first switching element and supplies this to the outside, and at the same time inputs the power supply output to the switching control circuit and compares this with a target level set within the upper and lower limit threshold value range, and controls the control signal so that the power supply output level approaches the target level, to stabilize the power supply output, there is provided a switch device in an output supply path for supplying the power supply output to the outside, and the switch device is driven by the normal verification signal of the monitoring circuit to thereby conduct the output supply path.
With such a construction, in the DC switching power supply unit, at the time of power supply start-up, the switch device is switched off so that the power supply output is not supplied to the outside. When the power supply output rises in the normal range, the switch device is switched on by the normal verification signal of the monitoring circuit so that the power supply output is supplied from the output supply path to the outside. Consequently, monitoring of the power supply output within the upper and lower limit threshold value range is possible, even if a start circuit is not provided.
The switch device is constructed with a first switch and second switch connected in series, and if the construction is such that the first switch is switched on after the second switch is switched on and switched off prior to the second switch, then the fail-safe characteristic is improved by the double system construction for the switch device. Furthermore, this construction is advantageous in the case that relay contact points where fusion faults must be considered are used for the switch.
With present invention, with an AC power supply unit incorporating an oscillator for generating an oscillating signal with input of the DC signal, and an output circuit having a switch circuit which is switchingly operated with the oscillation signal from the oscillator to generate an AC power supply output, the construction is such that supply of AC power supply output to the outside is made possible when a normal verification signal is being generated from the monitoring circuit. More specifically, the construction is such that when the normal verification signal is being generated, an oscillating signal from the oscillator is transmitted to the switch circuit. In this case, if the construction is such that a band-pass filter with a previously set predetermined frequency for the oscillator signal as a central frequency, is disposed in a signal supply path for supplying the oscillating signal from the oscillator to the switch circuit, then the verification of the power supply output frequency with the monitoring circuit can be omitted.
Furthermore, the construction may be such that the switching operation of the switching circuit is made possible when the normal verification signal is being generated.
Moreover, the construction may be such that a switch device is disposed in an output supply path for supplying the power supply output to the outside, and the switch device is driven by the generation of a normal verification signal from the monitoring circuit to conduct the output supply path and supply the power supply output to the outside. In this case, if the construction is such that the switch device has two switches connected in series, this construction gives a dual system with an improvement in fail-safe characteristic. Moreover, in the case where a relay contact point is used, the construction may be such that of the two switches, one switch comes on before the other switch and goes off after the other switch. Furthermore, the construction may be such that there is provided a switch off verification circuit for detecting an OFF condition of the switch device and outputting an off verification signal, and the monitoring circuit is able to generate a normal verification signal on the condition that the off verification signal is being generated.
The monitoring circuit incorporates; a level verification device for verifying that a voltage level of the power supply output is within the upper and lower limit threshold value range, a frequency verification device for verifying that the power supply output frequency is within a predetermined frequency range based on the pulse width of the power supply output, and a logical product device into which is input the output of the two verification devices, and the output from the logical product device is made the normal verification signal.
With such a construction, the monitoring circuit monitors the voltage level and frequency of the power supply output and when both are normal, a normal verification signal is generated from the logical product device.
The construction may be such that the frequency verification device detects the pulse width of the power supply output to verify the power supply output frequency. Moreover, the construction may be such that the frequency verification device filters the power supply output with a band-pass filter and detects that the output level of the band-pass filter is equal to or above a predetermined value to verify the power supply output frequency.
The monitoring circuit incorporates; a level verification device for verifying that a voltage level of the power supply output is within the upper and lower limit threshold value range, a frequency verification device for verifying that the power supply output frequency is within a predetermined frequency range based on the pulse width of the power supply output, a logical product device into which is input the output of the two verification devices, and a self hold device with an output from the logical product device input to a hold input terminal, and the off verification signal from the switch off verification circuit input to a trigger input terminal, which self holds a trigger input by its own output, and the switch device is driven based on the output from the self hold device.
With such a construction, when the switch device is in the OFF condition, a normal verification signal is generated so that the power supply output can be supplied, and an ON fault of the switch device can be checked. In this case, the construction may be such that a contact point which is operable from the outside is disposed in an off verification signal supply path for connecting to the trigger input terminal of the self hold device.
The monitoring circuit generates a normal verification signal on the proviso that one switch is off, and there is provided a switch drive device for intermittently switching the other switch off based on the normal verification signal, and a switch performance verification device for detecting ON/OFF operation of the other switch and generating a hold signal so that the normal verification signal continues and inputting this to the monitoring circuit, and the one switch is driven ON by the normal verification signal of the monitoring circuit.
With such a construction, the power supply output supply is carried out while verifying that the other switch is switching ON/OFF, and if this ON/OFF operation stops, the power supply output supply is stopped. Consequently, a fault of the switching device during power supply output supply can be checked.